In a small bowl stir together the crabmeat and tuna. Place a layer of bread cubes in the bottom of a 1 1/2 litre casserole dish. Cover with a layer of the crab and tuna, then a layer of cheese and a sprinkle of parsley. Repeat layers 2 more times ending with cheese and parsley on top.
In a medium bowl, whisk together the eggs, milk, butter, mustard powder and onion. Pour over the layers in the dish. Cover and refrigerate for at least 3 hours, or overnight.
Preheat the oven to 180 degrees C. Let the dish come to room temperature while the oven preheats.
Bake uncovered for 1 hour and 15 minutes in the preheated oven or until puffed and golden.